In-depth Interpretation of Home Energy Storage Inverter (Part I)

Types of household energy storage inverters

Residential energy storage inverters can be classified into two technical routes: DC coupling and AC coupling. In a photovoltaic storage system, various components such as solar panels and PV glass, controllers, solar inverters, batteries, loads (electric appliances), and other equipment work together. AC or DC coupling refers to how the solar panels are connected to the energy storage or battery systems. The connection between the solar modules and ESS batteries can be either AC or DC. While most electronic circuits use direct current(DC), solar modules generate direct current, and home solar batteries store direct current, many appliances require alternating current(AC) for operation.

In a hybrid solar energy storage system, the direct current generated by the solar panels is stored in the battery pack through the controller. Additionally, the grid can also charge the battery through a bidirectional DC-AC converter. The energy convergence point is at the DC BESS battery end. During the day, photovoltaic power generation first supplies the load (household electric products) and then charges the battery through the MPPT solar controller. The energy storage system is connected to the state grid, allowing for excess power to be fed into the grid. At night, the battery discharges to supply power to the load, with any shortfall supplemented by the grid. It’s worth noting that lithium batteries only supply power to off-grid loads and cannot be used for grid-connected loads when the power grid is out. In instances where the load power exceeds the PV power, both the grid and solar battery storage system can supply power to the load simultaneously. The battery plays a crucial role in balancing the energy of the system due to the fluctuating nature of photovoltaic power generation and load power consumption. Furthermore, the system allows users to set charging and discharging times to meet their specific electricity demands.

How a DC Coupled Energy Storage System Works

Hybrid photovoltaic + energy storage system

The solar hybrid inverter combines on and off grid functionality to enhance the efficiency of charging. Unlike on-grid inverters, which automatically disconnect the solar panel system during a power outage for safety reasons, hybrid inverters offer users the ability to utilize power even during blackouts, as they can operate both off grid and connected to the grid. An advantage of hybrid inverters is the simplified energy monitoring they provide. Users can easily access important data such as performance and energy production through the inverter panel or connected smart devices. In cases where the system includes two inverters, each must be monitored separately. DC coupling is employed in hybrid inverters to minimize losses in AC-DC conversion. Battery charging efficiency with DC coupling can reach approximately 95-99%, compared to 90% with AC coupling.

Furthermore, hybrid inverters are economical, compact, and easy to install. Installing a new hybrid inverter with DC-coupled batteries may be more cost-effective than retrofitting AC-coupled batteries into an existing system. The solar controllers used in hybrid inverters are less expensive than grid-tied inverters, while transfer switches are less costly than electric distribution cabinets. The DC coupling solar inverter can also integrate control and inverter functions into a single machine, resulting in additional savings in equipment and installation expenses. The cost effectiveness of the DC coupling system is particularly pronounced in small and medium off grid energy storage systems. The modular design of hybrid inverters allows for easy addition of components and controllers, with the option of incorporating additional components using a relatively inexpensive DC solar controller. Hybrid inverters are also designed to facilitate the integration of storage at any time, simplifying the process of adding battery packs. The hybrid inverter system is characterized by its compact size, utilization of high-voltage batteries, and reduced cable sizes, resulting in lower overall losses.